In order to efficiently accelerate a beam in an ion beam accelerator, it is necessary to remove electrons from ions to adjust into an intended charge state. A charge stripping film plays an important role for adjustment into an appropriate charge state by removing electrons from ions. Liquid lithium, beryllium, a carbon film, a carbon-boron composite film, a carbon nanotube composite film, and a carbon-organic composite film have been reported as charge stripping films (Non-Patent Document 1).
Since lithium reacts with water, a charge stripping film produced from liquid lithium requires a special apparatus in dry and a noble gas atmosphere, and the apparatus is very expensive and complicated (Non-Patent Document 1).
Meanwhile, beryllium has been frequently used as a target material due to its light element and high charge stripping efficiency (Non-Patent Document 1). However, beryllium is very expensive and toxic that caused to a lethal chronic disease called chronic beryllium disease when beryllium dust is absorbed into a human body.
Although vapor-deposited carbon film by arc discharge method or the like is used as a charge stripping film, the carbon film exhibits poor physical strength, low heat resistance or the like (Patent Document 1). Although a carbon-boron composite film has been reported as a method for increasing the physical strength of the carbon film, sodium impurities in the reagent during film fabrication are radioactivated by a beam irradiation (Patent Document 2).
Although a carbon nanotube (CNT) composite film (Patent Document 3) has high physical strength, the film can be damaged due to its lower heat resistance after a long-time operation. Thus, it is necessary to suspend the operation of the accelerator every time such damage occurs (Non-Patent Document 2). A carbon coated CNT film contained Iron and silicon impurities during film fabrication. For this reason, the charge stripping film after a beam irradiation is radioactivated, and several months are required for the charge stripping film to be transferable from a radiation controlled area. From these points of view, to develop a carbon-based charge stripping film for a charge stripping film with high quality and high heat resistance without radioactivation is important.
A carbon film using natural graphite is known as a carbon film with high quality and high heat resistance, (Patent Document 4). However, the carbon film is produced by press-working a powdery or scaly natural graphite as a raw material and the film strength of the carbon film is weak. Thus, the graphite fragments can be scattered inside the casing when the carbon film is damaged. In addition, the film thickness control is difficult, and hard to produce different thickness of thin graphite film, and the film has low density and rough variation in the film thickness.
When the carbon film is used as a charge stripping film for an ion beam, desired ranges of carbon film density and film thickness are predetermined depending on the charge state of the original beam and the desired beam, and the kind of the beam. The thickness of the film influences on the charge distribution and the charge efficiency of the beam (Non-Patent Document 1). Therefore, various thickness charge stripping films with sufficient heat resistance and strength for an ion beam charge stripping device have been demanded to meet the requirements of beam lines.